feat(Reborrow): Allow multiple lifetimes by aapoalas · Pull Request #163284 · rust-lang/rust

aapoalas · 2026-09-24T19:28:50Z

The Reborrow/CoerceShared borrow checking code is now robust enough to properly handle multiple lifetimes, and for that we can allow multiple lifetimes in the traits. This requires making the CoerceShared derive macro produce multiple lifetimes and in particular produce 'a_ where 'a: 'a_` additions to the generics, and removing some now-unnecessary checks from the wf-checks.

The derive trait code is some of the uglies stuff I've ever written.

r? @oli-obk

rustbot · 2026-09-24T19:28:54Z

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

oli-obk · 2026-09-28T10:25:58Z

@@ -803,9 +778,12 @@ fn field_tys_satisfy_relation_after_normalization_and_resolution<'tcx>(
            else {
                return false;
            };
-            if source_region != target_region {
-                return false;
-            }
+            infcx.sub_regions(
+                SubregionOrigin::RelateObjectBound(span),
+                target_region,
+                source_region,
+                ty::VisibleForLeakCheck::Yes,
+            );
            if ocx.sup(&cause, param_env, target_referent_ty, source_referent_ty).is_err() {
                return false;
            }


I couldn't find any in-tree docs about the lifetime handling. Should probably add some now that you aren't requiring equality anymore. I had to grab a variance table and poke at the code and tests here until I was sure it was all in the right direction 😆

View changes since the review

Where should they be located in? Just here in the code?

yea, there are existing docs in the code, but they don't mention lifetimes yet at all beyond the trivial usage

Added inline comments to this. You might want to also read https://hackmd.io/xl3EvNkZSHqqNi_K2XwdRg regarding this - if we do go back to an associated type on CoerceShared (which I think we should - otherwise we cannot represent &mut T and &T using Reborrow and CoerceShared alone) then the lifetime stuff here will revert to an invariant again :D

oli-obk · 2026-09-28T10:29:50Z

+
+    let Some(last) = path.segments.last_mut() else {
+        // FIXME(reborrow): we might want to support CoerceShared<Foo> for Bar at some point.
+        cx.dcx().emit_err(MalformedTarget { span: attr.span });


probably want to use more precise span for all the MalformedTarget errors, usually you have a specific item available which has its own span

View changes since the review

Added more specific errors.

rustbot · 2026-09-28T10:30:02Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

aapoalas · 2026-10-08T18:35:35Z

@rustbot ready

oli-obk

r=me with rebase and squashing

View changes since this review

rustbot · 2026-11-01T17:45:18Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

aapoalas · 2026-11-01T17:49:02Z

@rustbot ready

oli-obk · 2026-11-06T09:11:21Z

@bors r+ rollup

rust-bors · 2026-11-06T09:11:29Z

📌 Commit 500e0c6 has been approved by oli-obk

It is now in the queue for this repository.

@oli-obk

…ifetimes, r=oli-obk feat(Reborrow): Allow multiple lifetimes The `Reborrow`/`CoerceShared` borrow checking code is now robust enough to properly handle multiple lifetimes, and for that we can allow multiple lifetimes in the traits. This requires making the `CoerceShared` derive macro produce multiple lifetimes and in particular produce `'a_ where `'a: 'a_` additions to the generics, and removing some now-unnecessary checks from the wf-checks. The derive trait code is some of the uglies stuff I've ever written. r? @oli-obk

…uwer Rollup of 19 pull requests Successful merges: - #164784 (Introduce OptimizationObjective and fix the entry shim's size attributes) - #165037 (Drop array elements in order after a partial move) - #165078 (Use the `2.38` binutils version in CI) - #159374 (Lower #[no_mangle] to #[symbol_name]/#[link_name]) - #163284 (feat(Reborrow): Allow multiple lifetimes) - #164775 (Tweak output on misused `const` parameter to provide more context and more accurate suggestion) - #165194 (name elided lifetimes in `use<...>` suggestion when `impl Trait` params are present) - #165222 (Anonymize inner binders when checking refining) - #165335 (Fallible type ops) - #165388 (Accept variadic SysV64, Win64, and Aapcs FnAbis in rustc_public) - #165448 (Update `Vec::retain_unstable` docs to say that this method is generally faster than `Vec::retain_mut`) - #165494 (fix Miri ICE on exit with () return type) - #165503 (update Enzyme submodule) - #165505 (DFV: Reject `#[non_exhaustive]` on enum variants with default field values) - #165512 (Fix warnings for the L4Re targets) - #165514 (Remove unused `WhereEqPredicate` and `ScopeId` types) - #165518 (style-guide: Remove ABI explicitness prescription for extern items) - #165519 (Update books) - #165534 (rename MIR_borrow_checking timing scope to less confusing name)

Rollup of 15 pull requests Successful merges: - #165037 (Drop array elements in order after a partial move) - #165550 (Make `Literal::cstr_value` return a `CString`) - #159374 (Lower #[no_mangle] to #[symbol_name]/#[link_name]) - #163284 (feat(Reborrow): Allow multiple lifetimes) - #164775 (Tweak output on misused `const` parameter to provide more context and more accurate suggestion) - #165194 (name elided lifetimes in `use<...>` suggestion when `impl Trait` params are present) - #165222 (Anonymize inner binders when checking refining) - #165335 (Fallible type ops) - #165388 (Accept variadic SysV64, Win64, and Aapcs FnAbis in rustc_public) - #165448 (Update `Vec::retain_unstable` docs to say that this method is generally faster than `Vec::retain_mut`) - #165494 (fix Miri ICE on exit with () return type) - #165505 (DFV: Reject `#[non_exhaustive]` on enum variants with default field values) - #165512 (Fix warnings for the L4Re targets) - #165514 (Remove unused `WhereEqPredicate` and `ScopeId` types) - #165518 (style-guide: Remove ABI explicitness prescription for extern items)

@oli-obk

Rollup merge of #163284 - aapoalas:reborrow/allow-multiple-lifetimes, r=oli-obk feat(Reborrow): Allow multiple lifetimes The `Reborrow`/`CoerceShared` borrow checking code is now robust enough to properly handle multiple lifetimes, and for that we can allow multiple lifetimes in the traits. This requires making the `CoerceShared` derive macro produce multiple lifetimes and in particular produce `'a_ where `'a: 'a_` additions to the generics, and removing some now-unnecessary checks from the wf-checks. The derive trait code is some of the uglies stuff I've ever written. r? @oli-obk

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 24, 2026

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oli-obk reviewed Sep 28, 2026

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Comment thread compiler/rustc_hir_analysis/src/coherence/builtin/coerce_shared.rs Resolved

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dingxiangfei2009 reviewed Oct 1, 2026

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Comment thread compiler/rustc_builtin_macros/src/deriving/reborrow.rs Resolved

aapoalas force-pushed the reborrow/allow-multiple-lifetimes branch from 00337c0 to fb3a7fd Compare October 3, 2026 10:49

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cyrgani reviewed Oct 3, 2026

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Comment thread compiler/rustc_builtin_macros/src/deriving/reborrow.rs Resolved

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 8, 2026

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aapoalas force-pushed the reborrow/allow-multiple-lifetimes branch from 45b49ff to 467dff7 Compare October 17, 2026 20:26

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aapoalas force-pushed the reborrow/allow-multiple-lifetimes branch from 467dff7 to c39f3f2 Compare October 17, 2026 20:28

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oli-obk approved these changes Oct 23, 2026

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oli-obk added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 23, 2026

aapoalas force-pushed the reborrow/allow-multiple-lifetimes branch from c39f3f2 to 08a244e Compare November 1, 2026 17:45

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 1, 2026

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feat(Reborrow): Allow multiple lifetimes

500e0c6

aapoalas force-pushed the reborrow/allow-multiple-lifetimes branch from 08a244e to 500e0c6 Compare November 1, 2026 20:14

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 6, 2026

JonathanBrouwer mentioned this pull request Nov 6, 2026

Rollup of 19 pull requests #165548

Closed

Zalathar mentioned this pull request Nov 7, 2026

Rollup of 15 pull requests #165579

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rust-bors Bot merged commit db327f9 into rust-lang:main Nov 7, 2026
14 checks passed

rustbot added this to the 1.101.0 milestone Nov 7, 2026

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